This is NOT confirmed information. A lot of people saying you need to be level 20, However I was level 25 when it unlocked. So i'm guessing level 20 is the earliest it unlocks but it can unlock after that.
You'll get a notification telling you its unlocked, then you can phone Merryweather and pay $5000 to get a chopper. If you've unlocked it but it says unavailable, try shooting another player in your game.
Edit: Confirmed, this does not have a particular level it unlocks, all we know is level 20 is the earliest and it can unlock any level after that.